Major histocompatibility complex (MHC) class II molecule is an essential component of the immune response to foreign antigens. The MHC class II molecule is expressed on the surface of B lymphocytes, macrophages, dendritic cells, and activated T cells (Kaufman, J. F. et al. (1984) Cell 36:1-13). The MHC class II molecule presents a processed foreign antigen at the cell surface to receptors located on competent helper T cells (Lanzavecchia, A. (1985) Nature 314:537-539). Antigens are processed (proteolytically degraded into small peptides) and loaded onto the MHC class II molecules in an acidic, hydrolase-rich lysosome-like cellular compartment prior to transport to the cell surface (Peters, P. J. et al. (1991) Nature 349:669-676). Binding of the MHC class II/antigen complex to the receptor leads to activation of the helper T cells.
MHC class II molecules are heterodimeric glycoproteins composed of an .alpha.- and .beta.chain (Kaufman, J. F. et al., supra). Binding of an extracellular region of the MHC class II dimer by T cell receptors in the presence of processed antigen, or by anti-MHC antibodies, is thought to transduce a signal into the cell across the plasma membrane. Cross-linking of MHC class II molecules leads to increased intracellular cAMP levels and translocation of protein kinase C (PKC) to the nucleus in mouse cells (Cambier, J. C. et al. (1987) Nature 327:629-632), and to tyrosine phosphorylation, phosphatidylinositol turnover, increased free intracellular Ca.sup.2+, and PKC activation in human cells (Mooney, N. A. et al. (1990) J. Immunol. 145:2070-2076; Brick-Ghannam, C. et al. (1991) J. Biol. Chem. 266:24169-24175).
Signal transduction by cross-linked MHC class II molecules implies that an intracellular portion of the dimer plays an essential role and thereby contributes to activation of B cells and helper T cells during the T cell-dependent immune response. Experiments with cell lines expressing truncated forms of the mouse MHC class II molecule support this model and suggest that soluble cytoplasmic proteins interacting with the intracellular domain of the dimer participate in signal transduction (Ostrand-Rosenberg, S. et al. (1991) J. Immunol. 147:2419-2422).
A search for cytoplasmic proteins that interact with the intracellular domain of MHC class II molecules detects two proteins from separate genes that are designated PHAPI and PHAPII. These proteins interact specifically with the intracellular domain of the MHC class II .alpha.-chain, but not with the .beta.-chain or other unrelated proteins (Vaesen, M. et al. (1994) Biol. Chem. Hoppe-Seyler 375:113-126). Sequence analyses indicate that carboxy ends of PHAPI and PHAPII are highly acidic and are similar to the acidic activating domains of transcription factors (Vaesen, M. et al., supra; Ptashne, M. (1988) Nature 335:683-689).
PHAPI has two regularly spaced leucine/isoleucine motifs in the amino terminal region of the protein. These motifs are similar to those found in yeast adenylyl cyclase, human platelet receptor protein, and yeast mitosis dephosphorylation regulator (Vaesen, M. et al., supra; Li, M. et al. (1996) Biochemistry 35:6998-7002). PHAPI also has a nuclear localization signal in its carboxy terminus and significant amounts are detected both in the nucleus and diffusely distributed throughout the cytoplasm (Vaesen, M. et al., supra).
Additional analysis shows that PHAPI is identical to I.sub.1.sup.PP2A, a specific and potent inhibitor of the protein phosphatase 2A (PP2A; Li, M. et al., supra). PP2A is an important protein serine/threonine phosphatase that is involved in the regulation of diverse mammalian cell processes including reentry of quiescent cells into the cell cycle (Cohen, P. (1989) Annu. Rev. Biochem. 58:453-508; Shenolikar, S. and A. C. Nairn (1991) Adv. Second Messenger Phosphoprot. Res. 23:1-121; Mumby, M. C. and G. Walter (1993) Physiol. Rev. 73:673-699).
The discovery of polynucleotides encoding the human phosphatase inhibitor protein, and the molecules themselves, provides a means to investigate signal transduction, mitogenesis and cellular proliferation, and the immune response. Discovery of molecules related to phosphatase inhibitor protein satisfies a need in the art by providing new diagnostic or therapeutic compositions useful in the diagnosis, prevention, and treatment of cancer, AIDS, immunodeficiencies, autoimmune diseases, inflammatory diseases, and proliferative diseases.